Method for catalytic conversion of carbon monoxide in a hydrogen-containing gas mixture

ABSTRACT

A method for catalytic conversion of carbon monoxide with water to carbon dioxide and water in a hydrogen-containing gas mixture (carbon monoxide conversion) by passing the gas mixture over a shift catalyst that is at an operating temperature for the carbon monoxide conversion. The method is carried out with a shift catalyst based on noble metals that is applied to an inert support element in the form of a coating.

INTRODUCTION AND BACKGROUND

[0001] The present invention relates to a method for catalyticconversion of carbon monoxide with water to carbon dioxide and hydrogenin a gas mixture that contains hydrogen and other oxidizable components.

[0002] The conversion of carbon monoxide with water to carbon dioxideand hydrogen in the presence of catalysts is a known method forproducing hydrogen-rich gas mixtures, which is based on the followingexothermic reaction:

CO+H₂O⇄H₂+CO₂ ΔH>0  (1)

[0003] Here the following side reactions can occur: CO methanation: CO +3 H₂

CH₄+ H₂O ΔH > 0 (2) and CO₂ methanation: CO₂ + 4 H₂

CH₄ + H₂O ΔH > 0 (3)

[0004] The reaction in accordance with reaction equation (1) is calledcarbon monoxide conversion or CO conversion herein. The term “water gasshift reaction” is commonly used for this in the USA.

[0005] The production of hydrogen-rich gas mixtures from hydrocarbons,or alcohols, by steam reforming, partial oxidation or autothermicreforming is a known process. These gas mixtures (reformates) contain 1to 40 vol % carbon monoxide, depending on the method that is used.

[0006] To use the reformate as fuel in fuel cells, it is necessary toreduce the carbon monoxide contained in them as far as possible, inorder to avoid poisoning of the platinum-containing anode catalyst ofthe fuel cell in the oxidation of the hydrogen. In addition, theconversion of carbon monoxide in accordance with reaction equation (1)leads to an increase of the hydrogen content of the reformate and thusto an improvement of the efficiency of the overall process.

[0007] For reasons of size and weight, catalysts for conversion ofcarbon monoxide with very high activity and selectivity are required foruse in motor vehicles. The high space-time yields that can be achievedby this allow the volume of the reactors that are required to be keptsmall.

[0008] The known catalysts for the conversion of carbon monoxide havechiefly been developed for stationary industrial applications. Theemphasis lay in the production of pure hydrogen, ammonia and other largescale products that are based on the use of synthesis gas mixtures(CO/H₂). Catalysts for the conversion of carbon monoxide in accordancewith reaction equation (1) are also called shift catalysts herein.

[0009] These known catalysts are complete catalysts that containnon-noble metals. They are used in two-stage processes. In the firstprocess stage a so-called high temperature CO conversion (hightemperature water-gas shift, HTS) is carried out on Fe/Cr catalysts attemperatures between 360 and 450° C. In the subsequent second stage alow-temperature CO conversion (low temperature water-gas shift, LTS) isundertaken on Cu/ZnO catalysts at temperatures between 200 and 270° C.After the low temperature process stage carbon monoxide concentrationsof less than 1 vol % in correspondence with the thermal equilibrium areobtained.

[0010] The conventional catalysts for the conversion of carbon monoxidehave crucial disadvantages:

[0011] The described two stage conduct of the process is necessarybecause of the properties of these catalysts. While Cu/ZnO-containingcatalysts become deactivated above 270° C. because of recrystallization,or sintering, of the copper, the Fe/Cr-containing catalysts that areused in the high temperature range cannot be used at low temperaturesbecause of insufficient activity. If the indicated temperature range ofthe high temperature catalysts is exceeded, methanation reactions(reaction equations (2) and (3)) can occur, which reduce the selectivityof the high temperature catalyst and because of this lower the overallefficiency of the hydrogen generation system.

[0012] Both the known high-temperature and the low-temperature catalystsare bulk catalysts, in which the catalyst material is pressed to formpellets or other molded bodies. Accordingly, they consist entirely ofcatalytically active mass and are also called complete catalysts. As arule, they have a very high bulk weight.

[0013] The known industrial methods for conversion of carbon monoxide oncatalysts according to reaction equation (1) operate at space velocitiesof the gas mixture between 300 and 3000 h⁻¹. These low velocities arenot sufficient for use in motor vehicles.

[0014] High bulk weights and low space velocities lead to low specificconversion rates R_(CO) for the carbon monoxide, which is understoodwithin the scope of this invention to mean the amount of carbon monoxideN_(CO) converted per weight of the catalyst m_(cat) and reaction timeΔt. The weight of the catalyst here is given in grams, the reaction timein seconds and the amount of carbon monoxide in mol: $\begin{matrix}{R_{CO} = {\frac{n_{co}}{m_{Cat}\Delta \quad t}\left\lbrack \frac{{mo}\quad 1}{g \cdot s} \right\rbrack}} & (40)\end{matrix}$

[0015] The known Cu/ZnO and Fe/Cr catalysts have to be activated byreduction before they are used. The activated catalysts are sensitive tooxygen. Upon contact with atmospheric oxygen they are reoxidized anddeactivated in an exothermic reaction.

[0016] In comparison with the just described industrial high temperatureand low temperature catalysts based on Fe/Cr or Cu/ZnO, noble metalcatalysts for these uses are also known, mainly from the scientificliterature.

[0017] D. C. Grenoble et al. describe in “The Chemistry and Catalysis ofthe Water Gas Shift Reaction. 1. The Kinetics over Supported MetalCatalysts,” J. Catal. 67 (1980) 90-102, powdered catalysts that containCu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh or Ir as active componentsand that are deposited on aluminum oxide (Al₂O₃) as a support material.The kinetic tests gave a reaction order of about 0.2 for carbon monoxideand about 0.5 for the water that was used.

[0018] In “Methanization and Water Gas Shift Reactions over Pt/CeO₂,” J.Catal. 96 (1985), 285-287, Steinberg et al. observed poor selectivitiesin view of the carbon monoxide conversion according to reaction equation(1). Accordingly, the product gas mixture contains high proportions ofmethane.

[0019] In “Water gas shift conversion using a feed with a low steam tocarbon monoxide ratio and containing sulfur,” Catal. Today 30 (1996)107-118, J. Ross et al. investigate a Pt/ZrO₂ catalyst, in addition toFe/Cr, Cu/ZnO and Co/Cr catalysts. This catalyst shows a carbon monoxideconversion of 50% at 320° C. The Pt/ZrO₂ catalyst shows the highesttolerance for sulfur-containing compounds among the tested compounds. Itshows a conversion of 25% at 300° C. and a conversion of 70% at 350° C.This corresponds to a specific carbon monoxide conversion rate R_(CO)(300° C.)=7.00×10⁻⁶ mol/(g_(cat)·sec), or R_(CO) (350° C.)=1.95×10⁻⁵mol/(g_(cat)·sec).

[0020] FR 2567866 A describes a copper- and/or palladium-containingcatalyst on a support of ZnAl₂O₄ spinel, which is obtained byimpregnating the spinel formed into particles with diameters between 0.4and 0.6 mm with solutions of copper and/or palladium and calcining it. Aconversion of 86% is achieved with this catalyst at pressures of 40 barand a temperature of 250° C. at a very high excess of water (H₂O/CO=10).

[0021] The powdered catalyst systems that have been investigated in thescientific literature are not suitable for industrial use.

[0022] The known complete catalysts in the form of tablets, pellets orirregularly shaped particles are used as so called bulk catalysts. Onlyunsatisfactory space-time yields are obtained with such catalysts. Inaddition, the achievable specific conversion rates with these catalystsare low.

[0023] Accordingly, an object of the present invention is to provide amethod for conversion of carbon monoxide in a hydrogen-containing gasmixture that, under the conditions of mobile use in motor vehicles withtheir rapidly changing power requirements, a high specific conversionrate for carbon monoxide with good selectivity, has high temperaturestability and is insensitive to oxygen in the educt gas mixture.

SUMMARY OF THE INVENTION

[0024] This above and other objects of the invention can be achieved bya method for catalytic conversion of carbon monoxide to carbon dioxideand hydrogen (carbon monoxide conversion) in a hydrogen-containing gasmixture. For conversion of the carbon monoxide, the gas mixture ispassed over a shift catalyst, which is at the operating temperature forcarbon monoxide conversion. The method features a shift catalyst basedon noble metals that is applied to an inert carrier in the form of acoating.

[0025] The method of the present invention is specifically directed tomobile use in motor vehicles powered by fuel cells in order toeffectively removed carbon monoxide from the hydrogen-rich gas mixturethat is obtained by steam reforming, partial oxidation or autothermicreforming (hereinafter also called reformate gas) under all conditionsof operation of the motor vehicle. The gas mixture can contain up to 40vol % carbon monoxide, depending on its production.

[0026] The mobile use of the method imposes high requirements on itsefficiency and dynamics. During the operation of the motor vehicle, thecatalysts are loaded with very different space velocities. They varybetween a low space velocity at idling and 100,000 h⁻¹ at full load.

[0027] The method of the invention enables a high efficiency, i.e., ahigh space-time yield through the application of the catalyst in theform of a coating onto an inert carrier. Such a catalyst is also calleda coating catalyst herein. The monolithic honeycomb elements of ceramicor metal with cell densities (number of flow channels per area of crosssection) of more than 10 cm⁻² that are known from auto exhaust treatmentare suitable as carrier. However, metal sheet, heat exchanger plates,open-cell ceramic or metal foam elements and irregularly shaped elementsformed in each case according to requirements can also be used ascarriers. The thickness of the coating can vary between 10 and 100 μmaccording to application.

[0028] A carrier within the scope of this invention is characterized asinert if the material of the carrier does not participate orparticipates only negligibly in the catalytic conversion. As a rule,these are bodies with low specific surface and low porosity.

[0029] A catalyst that contains the elements of the platinum group ofmetals, thus platinum, palladium, rhodium, iridium, ruthenium andosmium, or gold as the catalytic active components on an oxide supportmade from the group consisting of aluminum oxide, silicon dioxide,titanium oxide, rare earth oxides or mixed oxides of these or zeolitesis suitable for the proposed method. In order to enable distribution ofthe catalytically active components on the support material that is asfine as possible, the support material should have at least a specificsurface (BET surface, measured in accordance with DIN 66132) of morethan 10 m²/g.

[0030] This noble metal catalyst exhibits a shift activity, i.e., it iscapable, if the appropriate reaction conditions exist (temperature, gascomposition), of converting carbon monoxide with water in accordancewith reaction equation (1) to carbon dioxide and hydrogen. For thisreason it is also called a noble metal shift catalyst herein. Its shiftactivity and selectivity can be improved by the addition of othercatalytically active components, or promoters. Among these are elementsof the rare earth metals, in particular cerium and lanthanum, as well asthe non-noble metals of the subgroups of the periodic system ofelements, especially iron or copper.

[0031] The shift activity and selectivity can, moreover, also beincreased by doping the support material with redox-active oxides of themetals cerium, zirconium, titanium, vanadium, manganese and iron in anamount of 1 to 50 wt % with respect to the total weight of the supportmaterial.

[0032] A preferred shift catalyst for the method in accordance with theinvention contains platinum and/or palladium together with iron orcopper as well as cerium oxide on a finely divided aluminum oxide.

[0033] The use of the shift catalyst based on noble metals for themethod also has the advantage that this catalyst does not becomedeactivated by contact with oxygen. For this reason no costly measuresto protect the catalyst from contact with air are necessary in a motorvehicle.

DETAILED DESCRIPTION OF INVENTION

[0034] The present invention will now be described in further detail.

[0035] In accordance with the invention, the described catalyst materialis not processed to complete catalysts, but rather is applied in theform of a coating to inert supports. In this way the disadvantages ofcomplete catalysts that consist of the catalytically active centers inthe interior of the complete catalyst being poorly accessible to thereactants are avoided in this method. Poor accessibility reduces thespecific conversion rate for carbon monoxide and thus the achievablespace-time yield. This has the corresponding negative effects on thevolume of the required reactor. The vibrations caused by operation ofthe motor vehicle additionally lead to undesired abrasion of completecatalysts, which blocks the flow paths in the catalyst bed and thuscontinuously increases the pressure difference in the reactor.

[0036] The process operates at gas mixture space velocities from idlingspace velocity up to a value of 100,000 h⁻¹ and at a pressure betweenatmospheric pressure and 10 bar, where the space velocity is given inreference to the volume of carrier coated with the catalyst. The methodcan be used both for low-temperature CO conversion as well a forhigh-temperature CO conversion.

[0037] A noble metal shift catalyst with an operating temperaturebetween 180 and 300° C. is used for the low-temperature CO conversion.The low operating temperature is achieved through a relatively highcharge of catalytically active noble metals on the catalyst. Inlow-temperature CO conversion the reformate gas usually contains 2 to 15vol % carbon monoxide and has an input temperature between 100 and 250°C. which results from the reforming process.

[0038] A noble metal shift catalyst with an operating temperaturebetween 280 and 550° C. is used for the high temperature CO conversion.In the high temperature CO conversion the reformate gas usually contains2 to 40 vol % carbon monoxide and has an input temperature between 300and 600° C., which results from the reforming process.

[0039] The method also allows a high temperature conversion stage and alow temperature conversion stage to be connected in succession. The gasmixture in this case leaves the high temperature stage at a temperaturethat corresponds to the operating temperature of the catalyst of thehigh temperature stage and for this reason has to be cooled to theoperating temperature of the catalyst of the low-temperature stagebefore contact with it.

[0040] There are various possibilities for production of a coatingcatalyst suitable for the method, a few of which are discussed here.

[0041] To produce a shift catalyst on a carrier element in accordancewith the invention, the support material for the catalytically activecomponents can be suspended in an aqueous solution of soluble compoundsof a noble metal selected from the group consisting of platinum,palladium, rhodium, ruthenium, iridium, osmium, gold and mixturesthereof and other soluble compounds of non-noble metals of thesubgroups. Then the acid suspension is neutralized at elevatedtemperature with a base, for example, a sodium carbonate, and thenreduced at the same temperature with an aqueous reducing agent(formaldehyde, hydrazine), filtered, washed, dried, calcined in anoxidizing atmosphere at temperatures between 300 and 550° C., and thenreduced at temperatures between 300 and 600° C. The catalyst material isagain suspended in water to produce a coating suspension. The carrierelement is coated with this suspension. For this, the methods forcoating carrier elements that are known from auto exhaust catalysis canbe used. To finish the production of the coating catalyst the coating isdried, calcined at temperatures between 300 and 600° C. and reduced in ahydrogen-containing gas at temperatures between 300 and 600° C.

[0042] As an alternative to the described method, the carrier element isfirst coated only with the support material, where the support materialcan contain rare earth oxides and oxides of non-noble metals of thesubgroups. The coating on the carrier element is then impregnated with asolution of at least one soluble noble metal compound, soluble compoundsof the rare earths and the non-noble metals of the subgroups. To finishthe production of the coating catalyst, the coated carrier element isdried, calcined at temperatures between 300 and 600° C. and reduced in ahydrogen-containing gas at temperatures between 300 and 600° C.

[0043] Another variation for making a coating catalyst in accordancewith the invention resides in first producing a suspension of thesupport material, the soluble compounds of the noble metals andoptionally the soluble compounds of the non-noble metals of thesubgroups and the rare earths. The dissolved components of thesuspension are then precipitated onto the suspended support materialthrough the addition of a basic precipitation agent such as sodiumhydroxide. The suspension prepared in this way is used directly forcoating the carrier element. To finish the production of the coatingcatalyst, the coated carrier element is dried, calcined at temperaturesbetween 300 and 600° C. and reduced in a hydrogen-containing gas attemperatures between 300 and 600° C.

[0044] The invention is illustrated in more detail by means of thefollowing examples.

EXAMPLE 1

[0045] A noble metal shift catalyst (catalyst A) was produced asfollows:

[0046] A ceramic element honeycomb carrier with 93 cells per squarecentimeter and a volume of 0.041 L was coated with 7.25 g γ-aluminumoxide by immersing it in an aqueous suspension of γ-aluminum oxide(specific surface 140 m²/g) and calcining for 2 h at 600° C. Aftercalcination the coated honeycomb element was impregnated with a solutionof Ce(NO₃)₂.6H₂O and then calcined for 2 h at 500° C. The calcinedmolded element was then impregnated with a solution of Pt(NO₃)₂,Pd(NO₃)₂ and Fe(NO₃)₃.

[0047] The catalytically active coating of the catalyst prepared in thisway had a total weight of 5.16 g, which corresponds to 126 g per literof volume of the honeycomb element. It contained 1.2 wt % Pt, 1.2 wt %Pd, 2.4 wt % Fe, 35.7 wt % CeO₂ and 59.5 wt % Al₂O₃.

[0048] The catalyst was tested under the conditions of a hightemperature conversion with a synthetic reformate. Its CO₂ selectivityS_(CO2), CO conversion, as well as specific conversion rate R_(CO) inaccordance with equation (4) were measured. The following gascomposition was used for the high temperature conversion: 27.0 vol % H2,9.0 vol % CO, 9.0 vol % CO₂, 18.0 vol % HO, 37.0 vol % N₂. The catalystswere tested at a gas space velocity GHSV=10,000 h⁻¹ and a pressure of 2bar (absolute).

[0049] The CO₂ selectivity S_(CO2) of the conversion of carbon monoxidewas calculated by means of the partial pressures of the carbon dioxideP_(CO2) and methane P_(CH4) that formed, as $\begin{matrix}{S_{{CO}_{2}} = \frac{P_{{CO}_{2}}}{P_{{CO}_{2}} + P_{{CH}_{4}}}} & (5)\end{matrix}$

TABLE 1 High-temperature CO conversion on catalyst A T [° C.]$\begin{matrix}S_{{CO}_{2}} \\\lbrack\%\rbrack\end{matrix}\quad$

CO Conversion [%] $\begin{matrix}R_{CO} \\\left\lbrack \frac{mol}{g_{cat} \cdot S} \right\rbrack\end{matrix}\quad$

300 100 27 3.0 · 10⁻⁵ 350 100 35 4.0 · 10⁻⁵ 400 100 45 4.8 · 10⁻⁵

COMPARISON EXAMPLE 1

[0050] A commercial Fe/Cr catalyst (catalyst B; tablets 5×5 mm) wastested under the same conditions as catalyst A. TABLE 2 High-temperatureCO Conversion on catalyst B T [° C.] $\begin{matrix}S_{{CO}_{2}} \\\lbrack\%\rbrack\end{matrix}\quad$

CO Conversion [%] $\begin{matrix}R_{CO} \\\left\lbrack \frac{mol}{g_{cat} \cdot S} \right\rbrack\end{matrix}\quad$

300 100 30 3.0 · 10⁻⁵ 350 100 37 4.0 · 10⁻⁵ 400 100 45 4.8 · 10⁻⁵

[0051] As Tables 1 and 2 show, both catalysts exhibit comparable COconversions. However, catalyst A in accordance with the invention showsa tenfold higher specific conversion rate R_(CO) in comparison withcatalyst B, because of its higher activity.

[0052] Further variations and modifications of the foregoing will beapparent to those skilled in the art and are intended to be encompassedby the claims appended hereto.

[0053] German priority application 100 13 895.0 is relied on andincorporated herein by reference.

1. A method for the catalytic conversion of carbon monoxide in ahydrogen-containing gas mixture with water to form carbon dioxide andhydrogen comprising passing said gas mixture over a shift catalyst,which is at an operating temperature for the carbon monoxide conversion,said shift catalyst being at least one noble metal that is applied to aninert support in the form of a coating.
 2. The method according to claim1, wherein the shift catalyst contains at least one of the noble metalsplatinum, palladium, rhodium, ruthenium, iridium, osmium and gold on anoxide support material selected from the group and consisting ofaluminum oxide, silicon dioxide, titanium dioxide, rare earth oxides,mixed oxides thereof and zeolites.
 3. The method according to claim 2,wherein the shift catalyst contains at least one rare earth metal as anadditional catalytically active component.
 4. The method according toclaim 2, wherein the shift catalyst contains at least one non-noblemetal of the subgroups of the periodic system of elements as anadditional catalytically active component.
 5. The method according toclaim 4, wherein the oxide support material is doped with a redox-activeoxide of a metal selected from the group consisting of cerium,zirconium, titanium, vanadium, manganese and iron in an amount of 1 to50 wt % with respect to the total weight of the support material.
 6. Themethod according to claim 5, wherein the shift catalyst containsplatinum and/or palladium together with iron or copper as well as ceriumoxide on finely divided aluminum oxide.
 7. The method according to claim1, wherein a honeycomb element of ceramic or metal, open-cell, ceramicor metallic foam elements, metal sheet, heat exchanger plates orirregularly shaped elements is a carrier.
 8. The method according toclaim 7, further comprising passing the gas mixture over the catalyst ata space velocity between an idling space velocity and 100,000 h⁻¹ and ata pressure between atmospheric pressure and 10 bar, where the spacevelocity refers to the volume of the carrier coated with the catalyst.9. The method according to claim 8, wherein the temperature of the shiftcatalyst lies between 180 and 300° C.
 10. The method according to claim9, wherein the gas mixture contains 2 to 15 vol % carbon monoxide. 11.The method according to claim 8, wherein the operating temperature ofthe shift catalyst lies between 280 and 550° C.
 12. The method accordingto claim 8, wherein the shift catalyst with an operating temperaturebetween 280 and 550° C. is another shift catalyst with an operatingtemperature between 180 and 300° C. and that the gas mixture is cooledto the operating temperature of the additional catalyst before contactwith it.
 13. The method according to claim 11, wherein the gas mixturecontains 2 to 40 vol % carbon monoxide.